1. Field of the Invention
The present invention relates to a sensor element which is fabricated on a semiconductor circuit by using a micromachining technology, as well as a manufacturing method thereof.
FIGS. 7A and 7B are schematic diagrams of a conventional sensor element disclosed in, for example, WO93/26051. In FIG. 7A, reference numeral 1 denotes a substrate, and 2 denotes a substrate-removing hole. An unillustrated sensing-element portion serving as a sensor element is provided in a region surrounded by the substrate-removing holes 2 on the substrate 1. Because the sensor uses a temperature change, a crystalline anisotropic etching technology based on a wet process using a solution is used so as to remove a substrate portion in a range surrounded by the substrate-removing holes 2 to form a cavity portion in the substrate below the sensing-element portion, whereby the substrate 1 and the sensing-element portion are separated and thermally isolated to prevent the heat from escaping to the substrate 1. With this sensor, it is ensured that the square region surrounded by the substrate-removing holes 2 is oriented in the (110) direction in the crystalline orientation of the substrate. In the crystalline anisotropic etching technology, the amount of substrate removal with respect to the (111) plane is small. Consequently, if the substrate is removed on the basis of this phenomenon, a desired region of the substrate which is principally surrounded by four (111) planes including the (110) direction is removed. In this way, a substrate portion which is desired to be left can be defined by the substrate-removing holes, thereby making it possible to fabricate an integrated sensor element.
However, when the substrate is removed in the above-described wet process, the erosion of the substrate at the (111) planes which are desired to be left is not nil, so that a region of the substrate which is larger than the region surrounded by the substrate-removing holes is removed. Particularly, when an attempt is made to form a highly integrated element, a semiconductor circuit for processing a signal from the sensing-element portion should be provided in the substrate in the vicinity of the sensing-element portion. Hence, there has been a problem in that if the region of the substrate larger than the region surrounded by the substrate-removing holes is removed, the breakdown of the circuit occurs, and the element fails to function as the sensor.
In addition, in the wet process, when the sensor element is removed the solution and is dried, the sensing-element portion may adhere to the bottom of the removed substrate portion and breaks down. To overcome this problem, a substrate removing technology based on a dry processing using xenon difluoride gas or the like has been proposed, as disclosed in WO96/23229. In the dry process, the drying step which is essential to the wet process is not used, there is an advantage in that the breakdown of the sensor is nil. However, since substrate removal occurs isotropically from the substrate-removing holes without conforming to the crystalline orientation, a region of the substrate larger than in the wet process is removed. Accordingly, there has been a problem in that the high integration of the sensor element is made more difficult than the wet process.